During the first intermoult of the sexual cycle, Jaera females show a cuticular ductus carrying the sperm to an internal receptaculum. A relationship exists between the respective lengths of the Cuticular organ and the male external reproductive apparatus. This organ is deciduous in most species. In J. albifrons and J. hopeana , the former cuticular organs are retained inside the female body. They are likely to allow sperm retention over several sexual cycles. The evolutionary implications of these features are discussed.